This invention relates to a printing sheet which is adapted for use in thermal transfer printing systems, particularly, a thermal transfer printing system using sublimable dyes. More particularly, it relates to a printing sheet which has both good sebum resistance and good writeability and also to a method for making the printing sheet.
As is known in the art, there have been widely used thermal transfer printing methods wherein an ink ribbon is heated according to image information by use of a thermal head or a laser beam to transfer, to a print sheet, an ink from the ink ribbon by thermal melting or diffusion or sublimation. In recent years, attention has been paid to so-called sublimation-type thermal transfer printing methods wherein full color images with a continuous tone or gradation are formed using thermally diffusable dyes such as sublimable dyes. For instance, attempts have been made to form images on a video printing sheet by selectively heating an ink ribbon according to signals of video images.
As a video image printing sheet, there is used a sheet substrate such as of polypropylene and a dye image-receiving layer formed on the substrate. The dye image-receiving layer is able to receive the dye transferred from an ink ribbon by heating and retains the resultant image thereon. The image-receiving layer has been conventionally made of resins which are susceptible to dyeing with dyes. Such resins include thermoplastic resins such as polyesters, polycarbonates, polyvinyl chloride, vinyl chloride copolymers such as vinyl chloride-vinyl acetate copolymers, polyurethanes, polystyrene, AS resins, ABS resins and the like.
Recently, in order to enhance sensitivity enough to form clear images and to improve the weatherability, light fastness and thermal stability of images so that once formed images can be stably kept, various attempts have been made on resins for the dye image-receiving layer. For instance, in order to improve the light fastness and weatherability of images, there has been proposed a dye image-receiving layer which is made mainly of cellulose esters (U.S. Pat. No. 5,278,130).
However, as the thermal transfer printing methods have been widespread, there is an increasing demand for satisfying the following characteristic properties, not to mention the improvements in the weatherability, light fastness and thermal stability. More particularly, there is a demand for realizing a printing sheet which has a good sebum resistance so that when a dye-receiving layer on which images have been formed is rubbed with a finger, the image is not attached to the finger. Moreover, there is also a demand for realizing a printing sheet which has such good writeability or writing properties that when the printing sheet is directly written on the dye-receiving layer thereof with oil base ink pens, the ink is stably fixed in the sheet.
However, known printing sheets cannot satisfy the requirements for both sebum resistance and writing properties. In order to improve the sebum resistance so as not to permit the finger having rubbed the image surface therewith to be attached with the dye, it is desirable to use a resin for the dye-receiving layer which is highly oil-repellent in nature to prevent the sebum from infiltration into the inside of the receiving layer. On the other hand, for the improvement in writing properties of a printing sheet which allows direct writing on the printing sheet by use of an oil base ink pen or marker, a resin for the receiving layer should favorably be oleophilic, not oil-repellent, unlike the case where the sebum resistance is improved. By this, a dye or an ink dispersing a dye therein can be infiltrated in the dye-receiving layer. Thus, the properties which are required for the resin in the dye-receiving and include a property for improving the sebum resistance and a property for improving the writing properties are contrary to each other. The improvements of both properties have been empirically difficult. For instance, where isocyanate crosslinking agents are incorporated in the dye-receiving layer for crosslinking reaction in order to improve the sebum resistance, an oily ink is unlikely to infiltrate into the dye-receiving layer, resulting in the lowering of the writing properties.